In order to achieve access independence and to maintain a smooth interoperation with wired terminals across the Internet, an IP multimedia subsystem (IMS) core network, specified e.g., in the 3GPP (Third Generation Partnership Project) specification TS 23.228, has been developed to be conformant to IETF (Internet Engineering Task Force) “Internet Standards”. The IMS enables network operators of mobile or cellular networks to offer their subscribers multimedia services based on and built upon Internet applications, services and protocols. The intention is to develop such services by mobile network operators and other third party suppliers including those in the Internet space using the mechanisms provided by the Internet and the IMS. The IMS thus enables conversion of, and access to, voice, video, messaging, data and web-based technologies for wireless users, and combines the growth of the Internet with the growth in mobile communications.
In this connection, it is also studied how interworking between 3GPP IMS and legacy circuit-switched (CS) core network domains of regular telephone users, such as users of second or third generation mobile networks, fixed public-switched telephone networks (PSTN) or fixed integrated services digital networks (ISDN), can be achieved. “Interworking” means that systems or components, possibly from different origins, work together to perform some task. Thus, interworking depends on standards to define the interfaces between the components. The term implies that there is some difference between the components which, in the absence of common standards, would make it unlikely that they could be used together. For example, software from different companies, running on different hardware and operating systems can interwork via standard network protocols.
A cellular network, i.e., a Public Land Mobile Network (PLMN) can be regarded as an extension of networks with CS domains and packet switched (PS) domains within a common numbering plan and a common routing plan. The PLMN infrastructure is logically divided into a core network (CN) and an access network (AN) infrastructure, while the CN infrastructure is logically divided into a CS domain, a PS domain and an IMS. The CS and PS domains differ by the way they support user traffic. These two domains are overlapping, i.e., they contain some common entities. A PLMN can implement only one domain or both domains. In particular, the CS domain refers to the set of all CN entities offering CS type of connections for user traffic as well as all the entities supporting the related signaling. A CS type of connection is a connection for which dedicated network resources are allocated at the connection establishment and released at the connection release. The PS domain refers to the set of all CN entities offering PS type of connections for user traffic as well as all the entities supporting the related signaling. A PS type of connection transports the user information using autonomous concatenation of bits called packets, wherein each packet can be routed independently from the previous one. The IMS domain comprises all CN elements for provision of IP multimedia services comprising audio, video, text, chat, etc. and a combination of them delivered over the PS domain.
The Session Initiation Protocol (SIP) is an application-layer control protocol which can be used in IP based networks, such as the IMS, for creating, modifying and terminating sessions with one or more participants. These sessions include Internet multimedia conferences, Internet telephone calls and multimedia distribution. Members in a SIP session can communicate via multicast or via a mesh of unicast relations, or a combination of these. In the full end-to-end use of SIP no specific requirements for interworking between non-SIP users, not having a SIP capable terminal equipment, have been considered.
On the other hand, CS networks may use a signaling protocol called Common Channel Signaling System #7 (more commonly called “SS7” or “C7”). In SS7 networks, ISUP (Integrated Services Digital Network (ISDN) User Part) signaling messages are used to set-up, manage and release calls between central office switches. ISUP messages also carry caller ID information, such as the calling party's telephone number and name. ISUP is used for both ISDN and non-ISDN calls between central office switches.
In 3GPP Rel-6 specification TR 23.903, re-dialing scenarios are defined to be performed by a terminal device (or user equipment (UE) in 3GPP terminology) when the called party does not support the requested video call, or when the call needs to swap between speech and video or vice versa. The UEs terminate the ongoing call and one of the UEs initiates a new call (i.e., redialing).
Additionally, 3GPP Rel-6 has produced a specification TS 29.163 for speech/audio interworking between IMS and CS networks.
However, when an IMS terminal tries to make a multimedia or video call to a CS terminal that is behind an ISUP leg, the IMS/CS interworking point (e.g., media gateway control function (MGCF)) will turn the request to a video-only request to the ISUP leg, because the ISUP network does not support downgrading of media compositions, i.e., there is no fallback mechanism in ISUP. The video call set-up will then always fail when the called terminal does not support video calls or when the called user does not want to receive a video call or when the terminating network does not support a video call.
More generally, when a terminal in a domain (second domain) supporting downgrading of the media composition (i.e., removing one or more of the requested or used media components) tries to make a multimedia call to another terminal in another domain (first domain) that does not support downgrading of the media composition, the domain interworking point will have to use a fixed media composition towards the second domain. The call set-up will then always fail when the called terminal does not support the requested fixed media composition or when the called user does not want to receive a call with the requested fixed media composition or when the terminating network does not support the requested fixed media composition.